Exemplary embodiments of the present invention relate generally to a method for monitoring the operation of a metal detection system and to a metal detection system that implements this method.
An industrial metal detection system is used to detect and reject unwanted metal contamination. When properly installed and operated, it will help reduce metal contamination and improve food safety. Most modern metal detectors utilize a search head comprising a “balanced coil system.” Detectors of this design may be capable of detecting metal contaminant types including ferrous, nonferrous, and stainless steels in a large variety of products such as fresh and frozen products.
A metal detection system that operates according to the “balanced coil”-principle typically comprises three coils that are wound onto a non-metallic frame, each exactly parallel with the other. The transmitter coil located in the center is energized with a high frequency electric current that generates a magnetic field. The two coils on each side of the transmitter coil act as receiver coils. Since the two receiver coils are identical and installed the same distance from the transmitter coil, an identical voltage is induced in each of them. In order to receive an output signal that is zero when the system is in balance, the receiver coils are connected in series with the second receiver coil having an inverse winding. Hence the voltages induced in the receiver coils, that are of identical amplitude and inverse polarity, cancel out one another in the event that the system, in the absence of metallic contaminations, is in balance.
As a particle of metal passes through the coil arrangement, the high frequency field is disturbed first near one receiver coil and then near the other receiver coil. While the particle of metal is conveyed through the receiver coils the voltage induced in each receiver coil is changed (by nano-volts). This change in balance results in a signal at the output of the receiver coils that can be processed, amplified and subsequently be used to detect the presence of metal contamination.
The signal processing channels split the received signal into two separate components that are 90° apart from one another. The resultant vector has a magnitude and a phase angle, which is typical for the products and the contaminants that are conveyed through the coils. In order to identify a metal contaminant, “product effects” need to be removed or reduced. Knowing the phase of the product, the corresponding signal vector can be reduced. Eliminating unwanted signals from the signal spectrum thus leads to higher sensitivity for signals originating from contaminants.
In order to obtain information about the sort and volume of the contaminants and in order to at least partially eliminate unwanted signals caused by “product effects” or disturbances such as vibrations, it is important that the system processes accurate signal amplitude and signal phase for the measured signals.
In the event that system deficiencies occur that degrade signal amplitude or signal phase, the measured results, which reflect the quality of the production process, are no longer reliable. For instance, the system may not raise an alarm if a contamination is present (false negative). Alternatively the system may raise an alarm if a contamination is not present (false positive).
Traditional methods, that verify that the metal detection system is correctly set up, do not allow detecting such system deficiencies.
Exemplary embodiments of the present invention are therefore based on providing an improved method for monitoring the operation of a metal detection system as well as on providing a metal detection system operating according to this method.
Particularly, exemplary embodiments of the present invention are based on providing a method that allows detecting malfunctions that would prevent the metal detection system from correctly detecting product contaminations.
More particularly, exemplary embodiments of the present invention are based on providing a method that allows avoiding false positive and false negative readings by the metal detection system.
The above and other advantages of exemplary embodiments of the present invention are achieved by an improved method for monitoring the operation of a metal detection system and a metal detection system operating according to this method.
An exemplary embodiment of the method serves for operating a metal detection system that comprises a balanced coil system with a transmitter coil that is connected to a transmitter unit, which provides transmitter signals having a fixed or variable transmitter frequency and with a first and a second receiver coil that provide output signals to a receiver unit. The coil system is designed and arranged in such a way that the output signals compensate one another in the event that the metal detection system is in balance.
According to an exemplary embodiment of the invention, a carrier signal having the transmitter frequency and a monitoring signal having a monitoring frequency are provided to a modulation unit that suppresses the carrier signal and provides a modulated monitoring signal, which is supplied to a monitoring coil that is inductively coupled with at least one of the receiver coils. The output signals of the receiver coils are further processed and demodulated in a demodulation unit that provides at least a demodulated monitoring signal, which is compared in phase and/or in amplitude with a reference, such as the phase and/or amplitude of the monitoring signal. In the event that the measured deviations in phase or in amplitude exceed a given threshold an alarm is raised indicating that a malfunction of the metal detection system has occurred.
An exemplary embodiment of the method allows for measurement of the performance of the metal detection system and verification if the measured performance lies within the specifications. It may be checked whether the transmitter part and the receiver part of the system operate correctly. Further, it can be checked whether other disturbances, such as influences from the installation site, e.g. vibrations or magnetic fields, have a negative impact on the metal detection system and the measurement process.
While the exemplary process of measuring the performance of the metal detection system may be intrusive and thus very reliable, any disturbing impact of this process on the measurement process is avoided. For this purpose the signals introduced into the balanced coil system may be selected in such a way that no interference with the measurement process occurs. The monitoring frequency may be selected to be above the frequency range of the product signals that are induced into the balanced coil system by objects measured during the operation of the metal detection system.
A decoupling of the monitoring signal from the measurement process may also be achieved by the placement and installation of the monitoring coil, which is preferably wound around a tail of one of the receiver coils that is attached to the receiver board. A possible remaining influence of the monitoring signal may be eliminated with the final calibration of the metal detection system. On the other hand, an exemplary measurement process may also not disturb the monitoring process since the measured products will not travel through the monitoring coil.
An example of the monitoring frequency is selected in the range of 50 Hz to 1000 Hz, preferably in the range between 500 Hz and 700 Hz. For example, a frequency of 615 Hz may be selected. Preferably, the monitoring frequency and the transmitter frequency are selected in such a way that the transmitter frequency, which may change during operation, is an even numbered multiple of the monitoring frequency. In this embodiment, the whole system may operate phase coherent allowing additional testing for phase coherence in the signal processing unit. The monitoring frequency may be derived from a reference unit that provides a reference frequency, which may be forwarded via a divider unit to a first input of the modulation unit. The divider unit may divide the reference frequency by an even number in order to obtain the monitoring frequency.
In a first basic embodiment, the carrier signal that is supplied to a first input of the modulation unit is derived from the output of a power amplifier that supplies the transmitter signal to the transmitter coil. In a second basic embodiment, the carrier signal is derived from a signal source, such as a frequency synthesizer, that provides reference frequencies to the power amplifier. The signal source may provide the transmitter frequency or a multiple thereof, which may be forwarded to another divider unit that provides the transmitter frequency, preferably including the in-phase and quadrature components. Deriving the frequency of the carrier signal and/or the frequency of the monitoring signal from the earliest stage of the system may allow for checking the system from the front end to the back end.
In an exemplary embodiment, since the carrier signal and the transmitter signal have the same frequency, the modulated monitoring frequency has a constant frequency offset to the transmitter frequency. This constant frequency offset corresponds to the monitoring frequency.
The monitoring process may therefore not be interrupted in the event that the transmitter frequency has been changed, e.g., because a frequency value has been found that is more favorable for the current measurement process.
For the process of obtaining a carrierless monitoring signal, the principles of double sideband modulation and single sideband modulation may be applied. For demodulation purposes, the carrier that is suppressed in the modulated monitoring signal may always be available from the transmitter unit.
In an exemplary embodiment, the modulation unit is a Single Sideband Modulator which provides terms for one sideband and terms for the carrier signal that are canceled due to their phase angle. For the remaining sideband, terms are provided that are in-phase. As a result, a single sideband with a frequency that is offset by the monitoring frequency from the transmitter frequency is supplied to the monitoring coil.
In an exemplary embodiment, the receiver coils are connected with one tail to one another and with the other tail to the respective tails of two identical center-tapped primary windings of a balanced transformer. The balanced transformer has two identical center-tapped secondary windings, whose opposite tails are connected to an amplifier. The amplifier forwards an amplified signal, preferably via a filter unit, to the demodulation unit, which provides in-phase components and quadrature components of the demodulated monitoring signal and preferably also the baseband signal or product signal originating from the products measured with the metal detection system.
For this purpose, the in-phase components and quadrature components of a reference signal with the frequency of the carrier signal, e.g., the carrier signal, are provided to the reference inputs of the demodulation unit, which delivers the in-phase components and the quadrature components of the demodulated monitoring signal, which are further evaluated in order to detect deviations of the amplitudes or of the phase of the resultant vector. In an exemplary embodiment, the demodulation unit provides the in-phase components and the quadrature components of the product signal as well.
In an exemplary embodiment, the monitoring signal may be used as a reference to evaluate the demodulated monitoring signal. For this purpose the monitoring signal may be supplied directly from the transmitter stage to the receiver stage.
In an exemplary embodiment, the in-phase component and the quadrature component and/or the related phase angle of the demodulated monitoring signal are compared with corresponding values that may be measured and stored during final testing of the metal detection system. Further provided may be at least one threshold value that should not be exceeded by a measured phase or amplitude during normal operation of the metal detection system. However, in the event that a threshold is exceeded the signal processing unit may raise an alarm indicating a malfunction of the system.
In addition to the novel features and advantages mentioned above, other benefits will be readily apparent from the following descriptions of the drawings and exemplary embodiments.